Materials wherein a matrix of plastic has been reinforced by inserted fibers have been used for the production of orthopedic devices for some time. For various reasons, plastic materials are better suited than the previously used customary materials such as plaster, leather or metals, because they are lighter, more permanent, less sensitive to moisture, more compatible with the skin and can be more easily made into the desired shape.
Up to now, thermosetting resins have generally been used as materials for the matrix of orthopedic devices. The disadvantage of these materials for orthopedic devices, with and without fibers, is that even though they can be made into the shape desired for the orthopedic device without problems, after curing they cannot be plastically deformed again. In this way such orthopedic devices are no longer optimal, because it is not possible to perfect their shape after experimental use or to adapt them to anatomical changes or new requirements over time.
For this reason, a material which was constructed in a sandwich-like manner and had outer layers of a thermosetting plastic material, between which a core of thermoplastic material was located, was also used for producing orthopedic devices which can be re-deformed. When shaping these materials, the thermosetting outer layers are only elastically deformed, while the core is plastically deformed. The plastically deformed core then maintains the elastically deformed outer layers in a pre-stressed position. Re-deformation is always possible by again heating and plastically deforming the core, in the course of which it is necessary to take the elastic limits of the outer layers into consideration.
However, orthopedic devices made of this sandwich-like material also have a considerable disadvantage. Further tension, caused when using the orthopedic devices, is added to the tension caused by shaping of the outer layers. The sum of these tensions must, of course, not exceed the permissible tension of the material, as a result of which only comparatively low tension in use can be permitted.
It must furthermore be taken into consideration that because of their sandwich-like configuration the layers of material form a shear connection with a high degree of rigidity with a high modulus in flexure E. Since it is generally true in the elastic range that deformations, i.e. displacements f, correspond to the quotient of tension/modulus of flexure, a high modulus of flexure in any event results in small displacements. It is furthermore true that in the same way that the sum of shaping tension and use tension is not allowed to exceed the permissible tension, the sum of shaping displacement f(D) and the displacement by use f(G) is not allowed to exceed the permissible displacement f(ZUL). Therefore, EQU f(D)=f(XUL)-f(G).
applies for the just permissible shaping displacement.
Because of these relationships of the deformations, only a small shaping displacement f(G) is the result--depending on the necessary displacement by use f(G)--, which may be undesirable, depending on the type of the orthopedic device.
For this reason it was attempted to employ fiber-reinforced, partially crystalline thermoplastic materials for producing orthopedic devices. In contrast to the sandwich-like materials, partially crystalline thermoplastic materials permit sufficient deformation by use. In addition they also can be plastically deformed repeatedly, in contrast to the above mentioned thermoserring plastics, if they are heated to the appropriate temperature. However, they have two disadvantages which make their use problematical, particularly for orthopedic devices. First, the partially crystalline thermoplastic materials must be pressed after deformation, which is hardly possible with orthopedic devices; and secondly, the temperature range in which these plastic materials act in a thermoplastic manner is very narrow. If the temperature falls below this range, the plastic material can no longer be plastically deformed, if the temperature rises above above this range, the material melts. The upper limit and the lower limit of the temperature range only differ by approximately 5.degree. C. It is easy to understand that staying within this temperature is hardly possible, so that the material cannot be plastically deformed in a simple manner.